1. Field of the Invention
The present invention relates generally to a fuel supply system and more specifically to a system which monitors one or more operational parameters of the engine, determines the nature of the fuel being combusted and then subsequently controls the fuel supply in accordance with the analysis in a manner which optimizes transitional engine operation.
2. Description of the Prior Art
It has been previously proposed to sense engine knock, and to determine, using this in combination with other engine operational parameters, the octane rating of the fuel being combusted and to modify the ignition timing etc., on the basis of the octane analysis. An example of this type of system is found in U.S.P. No. 4,630,584 issued on Dec. 23, 1986 in the name of Higashiyama et al.
However, this type of system only utilizes octane rating and driving conditions and does not take the other influencing factors into account. That is to say, the fuel supplied into the induction system by SPI or MPI systems, for example, separates into two distinct fractions before reaching the combustion chamber. Viz., some of the fuel assumes a gaseous form and is entrained in the air flow per se while the remainder remains in a liquid state and forms a film which flows along the walls of the induction system conduits. Depending on the composition of the fuel, the fraction which volatilizes and enters the combustion chamber or chambers of the engine in a gaseous state and the fraction which flows along the walls of the induction system, varies considerably. For example, if the fuel contains a relatively large highly volatile fraction, the amount of liquid running on the walls of the induction system conduiting will tend to be reduced as compared to the amount which is gasified and entrained in the air flow; and vice versa. Further, if the fuel consists of gasoline mixed with an amount of alcohol, it is necessary to supply a larger amount of fuel in order to produce the required power output as well as take the effect of the alcohol on the gassified/wall flow (liquid) fraction balance into account. For example, as shown in FIG. 29, in the event that the engine is operating under steady state conditions and the driver depresses the accelerator in order to accelerate the vehicle, the amount of air inducted into the engine rises sharply as shown in trace 29A. In response to this the amount of fuel supplied into the induction system can be increased in a manner to essentially parallel the change in air flow (see trace 29B). However, there is a transitional period wherein this measure does not maintain the desired A/F. As shown in traces 29C to 29D, the time required for the more volatile fraction of the fuel (referred to hereinafter as "light" fuel) and the less volatile fraction (hereinafter referred to as "heavy" fuel) to actually reach the combustion chambers varies markedly. Viz., the gassified and wall flow fractions of the light fuel are able to reach and enter the combustion chambers in a time tL while in the case of the heavy fuel, the time require for the corresponding fractions to reach the same requires a time tH.
Thus, during the initial period of transitional modes of engine operation it is clear that the air-fuel ratio of the air-fuel mixture entering the combustion chambers can fluctuate widely from the intended value depending on the composition of the fuel.
In more specific terms, in the case of electronically controlled fuel injected engines the fuel injection control signal pulse width is calculated using the following equation: EQU Ti=Tp(COEFF+Kacc)+ALPHA+Ts (1)
wherein:
Tp denotes the basic pulse width; PA1 Kacc denotes a basic correction factor which varies with the amount of acceleration demanded; PA1 COEFF denotes a compound correction factor which takes into account a number of variables other than acceleration; PA1 ALPHA denotes a feed-back control air-fuel ratio correction factor; and PA1 Ts denotes the rise time required for the injector to actually begin injecting after the control signal is applied to thereto.
If the correction factors are fixed and selected to suit a light fuel content, the accuracy thereof lowers as the content of the heavy fuel increases and the time required for the fuel to actually reaches the combustion chamber increases from time tL toward time tH; and vice versa. As the accuracy of the correction factors lowers, so does the accuracy of the injection control. This results in poor A/F control during acceleration and similar transitional modes of engine operation inducing an attendant loss of acceleration characteristics and fuel economy.
Further, during engine start-up the wall flow fraction increases due to the inherently low engine temperatures and reduced vaporization characteristics. Again, if the correction factors are predetermined in a manner which optimizes for light fuel, as the fraction of heavy fuel increases injection control accuracy is lost to a notable degree and thus invites a loss of fuel economy and emission control.
Thus, in order to accurately control the supply of fuel to the engine during transient modes of operation, it is necessary to know the composition (as different from the octane rating) of the fuel which is currently being injected or otherwise being supplied into the engine so as to enable the desired level of injection control to be maintained. However, until this time a technique via which the analysis of the fuel content can be carried out in situ and in close to real time has been lacking.